That type of food poisoning is fast acting, a type of staph basically and you get sick from it's toxins. It also doesn't cause an off taste. Toxins produced also aren't necessarily killed by reheating.

Regarding the lawsuit, the caterers lawyers would have to prove they followed food safety rules first and foremost. Then they would likely have to try and establish where contamination took place. It's usually introduced by person to food contact, so maybe they could present enough doubt as to whether it was an employee or guest. Kind of a Longshot

I would think either direction would be tough to prove three years after the fact. Hopefully the caterer maintains good records and didn't throw them away.
Food poisoning can present as early as 20 minutes after a meal, depending on the type of infection.